1. Field of the Invention
The invention relates to a method of operating a steel converter which has a basic wear-lining formed of refractory material at least partly composed of burnt magnesite including the step of performing maintenance on the said lining by intermittently spraying refractory material onto the wear-lining and/or slagging-in while the wear-lining is hot.
The invention also relates to a method of providing and maintaining such a wear-lining and to a steel converter which is provided with such a wear lining.
2. Description of the Prior Art
In this specification and claims, we use the term "steel converter" to refer not only to ladles used in steel-manufacture by the so-called LD-process but also to the vessels used in all other processes in which a mass of molten metal is refined to steel by blowing in or blowing on oxygen. Such vessels are usually provided with a so-called safety or permanent lining which is formed as brickwork on the steel wall of the vessel, and with a wear-lining which is contacted by the molten metal. It is common to reduce the rate of wear of the wear-lining, and thus increase the useful life of the lining, by performing a maintenance step on the wear-lining regularly after a certain number of charges of the converter. This step is either spraying a refractory mass onto the wear-lining or to coat the wear-lining with slag, in both cases while the lining is hot. The latter practice, so-called slagging-in, typically consists in that any residual slag left in the converter after steel tapping is distributed over the surface of the wear-lining by tipping the ladle. However, methods of distributing residual slag over the wall surface by spraying this slag are also known. None of these methods need any further explanation since they are sufficiently widely known in this technical field.
Although in the past, and even at present in Western Europe in particular, use is frequently made of bricks of burnt dolomite in the wear-lining, there is a marked trend to change over to bricks manufactured of burnt magnesite. It is generally accepted that the use of the latter material may result in prolongation of the useful life of the lining. Bricks of magnesite are obtained by pressing a starting material of burnt magnesite grains, mixed with coal tar products as a binding material, into bricks. Bricks are also sometimes made of magnesite grains which are burnt at high temperatures so that ceramic cohesion between the grains is achieved, whereafter the bricks are impregnated with coal tar products.
The burnt magnesite particles or grains used for the manufacture of such bricks consist chiefly of magnesium oxide, while other oxide components such as CaO, SiO.sub.2, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, etc. may also be present to a greater or lesser degree. In mineralogical terms, the magnesite is composed of so-called periclase particles, the size of which typically varies between 0.01 and 0.1 mm. The other oxides mentioned above have various mineralogical combination types, which are principally of the periclase type. On account of their chemical affinity, the oxides CaO and SiO.sub.2 preferentially form compounds with each other, whether or not in combination with MgO.
G. R. Rigby, H. M. Richardson and F. Ball in "Bulletin of the British Refractories Research Association" No. 71 of June 1946, describe a method by which the mineralogical composition of sintered magnesite can be calculated on the basis of its chemical composition. This method, which has been confirmed by experiments, proposes that the ratio of the CaO and SiO.sub.2 oxides present is determinative of the type(s) of silicate produced. In a molar CaO-SiO.sub.2 ratio of 2 or more, these oxides form di-calciumsilicate or tri-calciumsilicate. If this molar ratio is between 1.5 and 2, the compounds di-calciumsilicate and merwinite (3CaO.MgO.2SiO.sub.2) are produced; if between 1 and 1.5, the compounds merwinite and monticellite (CaO-MgO.SiO.sub.2) are produced; if the molar ratio is less than 1, monticellite and forsterite (2MgO.SiO.sub.2) will be produced.
Generally in this technical field it is assumed that the longest life of the wear-lining can be achieved by using magnesite material of the highest possible grade. In this respect a high-grade magnesite material is assumed to be characterised by a maximum percentage of MgO and by a suitably chosen molecular ratio of CaO and SiO.sub.2.
The ratio of CaO to SiO.sub.2 in the magnesite is important particularly in those magnesite grades which contain large quantities of oxide other than MgO. This is the reason that, in the use of magnesite as a lining material for steel furnaces and in particular converters, in general the quality of the magnesite grade is assumed to be higher as the MgO content increases, and the requirement has been set that the molecular ratio CaO:SiO.sub.2 is greater than 2.
When the amount of dirt present is low, the periclase particles will be in direct contact and form a directly bonded structure, but if a high percentage of dirt is present, contact between the perclase particles is via an intermediate phase. If this intermediate phase is low-melting, the obvious assumption is that under operating conditions the wear resistance of the material will be harmed.
Recent developments have shown that the ultimate lifetime of the lining brickwork depends more and more on the extent to which attempts to apply maintenance material onto the wear-lining are successful.